1. Field of the Invention
The present invention relates to a lock-up type automatic transmission, viz., an automatic transmission having a lock-up clutch and a control.
2. Description of the Prior Art
Automatic transmissions provided with a torque converter are known. The torque converter has a pump impeller driven by an engine to rotate operating oil within the torque converter wherein the rotation of the operating oil causes a turbine runner to rotate under the reaction of a stator thereby to multiply the torque (torque converter state). During the operation of the torque converter there is a slip between the pump impeller and the turbine runner and therefore the automatic transmission provided with such torque converter has an advantage of easy operation, but has a drawback of poor power transmission effeciency, leading to poor fuel economy.
To alleviate this drawback, there has been proposed a so-called a torque converter with a lock-up clutch wherein during a relatively high vehicle speed operation range when torque variation of the engine does not create a problem, the turbine runner is directly connected to the pump impeller (lock-up state) thereby to eliminate the slip therebetween, and a lock-up type automatic transmission provided with a torque converter of this kind is already employed in some vehicles.
If a lock-up clutch of a torque converter is engaged when a vehicle speed is above a predetermined value in each of gear ratios, the lock-up operating ranges for all of the gear ratios, respectively, may be illustrated as shown in FIG. 7. This figure shows a shift pattern diagram for an up-shift operation of a three forward speed automatic transmission, wherein V.sub.1, V.sub.2 and V.sub.3 denote predetermined lock-up vehicle speed values for a first gear ratio, a second gear ratio, and a third gear ratio, respectively, and A, B and C denote lock-up ranges for the first gear ratio, the second gear ratio, the third gear ratio, respectively. In the case of the automatic transmission wherein the lock-up clutch is engaged whenever the vehicle speed exceeds a predetermined vehicle speed value in each of the gear ratios, when the vehicle undergoes an automatic gear shifting operation with the accelerator pedal depressed to a great degree (a great opening degree of the throttle), the torque converter remains in the lock-up state during gear shifting operation because the lock-up ranges A to C adjoin one after another. However, if the gear shifting is effected with the torque converter remaining in the lock-up state, the torque variation is not absorbed by the torque converter, thus allowing a great shock to take place upon gear shifting.
Accordingly, it is designed to release the lock-up state during gear shifting operation even when the vehicle is operations in one of said lock-up ranges so as to allow the torque converter to operate in the torque converter state. For this purpose, there is provided a shifting operation detector circuit which produces a shift signal having a predetermined time duration in response to and after the appearance of a command for the shifting between two gear ratios to temporarily release the lock-up state during the actual gear shifting operation.
However, during the shifting operation from the second gear ratio to the third gear ratio, since the shift signal having the predetermined time duration T.sub.1 is produced simultaneously with the instant t.sub.1 when the demand is made to release the lock-up state, the lock-up state is released too early. This causes inconveniences as follows. There is a time lag from the instant when the shifting demand is made to the instant when the actual shifting operation initiates, viz., the initiation of actuation of the friction elements, because of the existence of a response delay in the hydraulic control system of the transmission. Therefore, if the lock-up state is released simultaneously with the shifting demand, since the lock-up state is released before the actual shifting operation initiates, the speed of the engine increases rapidly during the moment ranging from t.sub.1 to t.sub.2 as shown in FIG. 6(A), causing the engine to race. Owing to the fact that the release of the lock-up state terminates at an instance during the moment from t.sub.3 to t.sub.4 when the shifting operation is completed, the torque converter does not absorb the shift shock which occurs upon the gear shifting operation, and this fact, cooperating with the fact that the engine speed has been increased due to the engine racing mentioned above, increases the magnitude of a peak torque at the instant t.sub.5 right after the shifting operation, with the inevitable result that a large shift shock occurs.
This tendency becomes marked during up-shifting operation in the automatic transmission because the shifting operation takes place with the power-on mode, and this problem is more serious upon up-shifting from the second gear ratio to the third gear ratio when a front clutch to be engaged for the third gear ratio is engaged while releasing a second brake to be engaged for the second gear ratio so that a relatively long overlap time is necessary.
As one measure to solve this problem, it is conceivable to increase the duration time T.sub.1 up to the instant when the shifting operation will be completed, but this leaves the racing problem of the engine unsolved. This increase in the engine speed causes the corresponding increase in the magnitude of the shift shock.